The present invention relates to molds and to the processes of molding treads. In particular, it relates to molds and the processes for molding tires whose tread comprises indentations, channels or other recesses extending substantially transversely to the tread and which open onto the lateral faces of the tread. These recesses do not open, or do not fully open out at the surface of the tread when this is new. This is known as an evolutionary sculpture tread, because as the tread wears and the xe2x80x9csubterraneanxe2x80x9d or xe2x80x9csubsurfacexe2x80x9d channels appear, the visible and therefore active profile of the tread evolves. An advantage of such treads is for example to give substantially constant performance throughout the normal service life of the tire. The patent application WO98/54009 describes such notions.
These particular forms of tread can be realized with various molding processes linked to various molding techniques. When such processes are implemented industrially, two major problems have to be overcome: on the one hand, to make the molding successful, i.e. to create without defect (visible or otherwise) a tire comprising axial channels in the tread; and, on the other hand, to permit ejection, i.e. separation, of the molded tire from the elements constituting the mold without the risk of damage either to the tire or to the tooling.
The document WO98/54009 describes such a process and a mold for this process. The axial channels of the tread are molded by fingers integral with the shells provided for molding the sides of the tire. During molding, the internal pressure on the uncured (hence moldable) tire exerted by the membrane causes the outer diameter of the uncured tire to expand. This stage, known as xe2x80x9cshaping,xe2x80x9d allows the material forming the uncured tread to fill all the spaces in the mold to create the final tread comprising the various grooves and recesses which can be seen to open at the running surface or at the shoulders. In order to remove the tire from the mold, the shells are removed axially, which allows the fingers to be extracted from the tread. The segments of the crown ring, which mold the running surface, are removed radially in order to allow their projecting parts (the projections which mold the sculpturing) to disengage from the tread of the molded tire.
One problem with this process is that it is, of course, limited to manufactures in which shaping is relatively extensive, because the shaping must correspond at least to the thickness of the intended sculpture, including the subjacent channels. Indeed, since the fingers are carried by the shells for molding the sides, their radial position is fixed. In order to be loaded into the mold, the uncured tire must therefore have an outer diameter which allows it to be positioned (radially) within the fingers. Then, the diametric expansion (or shaping) brought about, for example, by the inflation of a vulcanization membrane must be sufficient to make the molding material penetrate around the fingers and into all the spaces in the tread.
However, industry strives to limit to the maximum the size of shaping, and this is for at least two reasons. The first reason is the objective of greater uniformity of the molded tires. In fact, the greater the degree of shaping, the more difficult it is to obtain regular distribution of the molding material that flows when the uncured tire is subjected to pressure. The second reason is the use of more and more rigid (inextensible) forms of tire architecture in order to improve certain tire performances (e.g. at high speed). These forms of architecture are consequently less and less capable of extension, hence of shaping.
Another known process (described for example in U.S. Pat. No. 1,733,064) consists in inserting fingers or pins axially into the uncured rubber of the tread. This method then encounters other limitations: the geometry and resistance of the fingers or pins must permit their penetration without risking damage to the tool but also without moving or deforming the uncured tire in an unwanted manner. In practice, this technique is therefore restricted to fingers having a simple geometry and sufficient thickness to resist buckling. On the other hand, the kinematics necessary to control the movements of these fingers also imposes a considerable restriction on the possible forms.
An object of the invention is therefore a mold and process which permit both low-shaping or no shaping manufacturing and which would not suffer from the above-mentioned limitations.
This object is achieved by a mold for a tire tread having a running surface, lateral surfaces, radial recesses opening at the running surface and axial recesses opening on to at least one of the lateral surfaces, the mold comprising a first plurality of crown sectors for molding the radial recesses, a second plurality of shoulder sectors, among which at least some carry projections for molding the axial recesses, the crown sectors being radially movable between an open configuration permitting loading of uncured tire into the mold and a closed configuration permitting molding of the tire, the mold being characterized in that the shoulder sectors are radially movable between the open configuration and the closed configuration independently of the crown sectors.
Indeed, the mold according to the invention, by the radial mobility of the elements molding the channels or transverse subterranean recesses, permits simultaneously loading of the uncured tire without constraint due to the presence of these elements, low or zero shaping molding, and problem-free ejection from the mold.
Preferably, the projections carried by the shoulder sectors take the form of fingers and extend substantially axially projecting from said sectors and preferably over a distance of more than 10%, or preferably even more than 25% of the width of the crown sectors.
In a preferred embodiment, the fingers bear, during the closing movement, on the crown sectors in order to resist the pressure exerted by the molding material on the fingers during closing of the mold/and or during shaping.
In a particular embodiment, the fingers extend in a generally oblique direction relative to the axis of the tire. In this case, the second plurality of shoulder sectors is preferably capable of moving in rotation about the axis of the tire in order to ease ejection from the mold.
The invention also relates to a process of molding a tread comprising a running surface, lateral surfaces, axial recesses opening axially (in the width direction) on to at least one of the lateral surfaces, the process comprising the steps of inserting into the uncured tread projections which mold the axial recesses, and then extracting the projections from the vulcanized tread, the projections being inserted in the direction of the thickness of the tread and extracted in the direction of the width of the tread. The act of inserting the projections into the uncured rubber, at least in part by a movement in the thickness direction of the tread, which is independent of any shaping, makes it possible to reduce or even render superfluous the process of shaping. The act of then extracting the projections in the direction of the width of the molded tread (i.e. in the direction of the axis of the tire for which the tread is intended) makes it possible to eject the tire without damage, even when the length of the projections is considerable.
Preferably, the process according to the invention uses a mold such as described above and comprises successively the following steps:
Inserting an uncured tire into the mold in an open configuration,
Closing the mold by centripetal motion of the crown sectors and shoulder sectors,
Molding the tire,
Opening the mold by radial motion of the crown sectors in order to eject the radial recesses, the shoulder sectors remaining substantially in a closed configuration, and,
Continuing opening of the mold by axial motion of the shoulder sectors.
Preferably, in accordance with this process, the diameter of the uncured tire is less than the diameter of the molded tire by a value xe2x80x9cd,xe2x80x9d which is less than 1% of the diameter of the molded tire.
The invention will be explained more fully with the aid of a description of the drawings.